The present invention relates generally to catalysts of the production of diethyl oxalate from carbon dioxide and, more specifically, to highly effective palladium catalysts promoted with cerium.
Ethylene glycol (EG) is a crucial raw material with a global demand of around 25 million tons each year, which is mostly produced through traditional petrochemical technology.[1, 2] However, the cost of this production is relatively high due to the continuous increasing price of natural gas and crude oil, and dwindling sources of petroleum. Furthermore, strong acids or alkalis such as sulfuric acid or sodium hydroxide have to be used through the traditional method, which causes severe corrosion to the equipment and environmental problems.[3] Therefore, a green route which is independent of petroleum while achieves high yield of EG is in demand and of great significance.
Coal is the most abundant energy reserve in the world that some people like because of their needs while others hate due to the various emissions resulting from its combustion.[4] To reduce CO2 emission and produce high-value fuels and chemicals from coal, coal gasification and liquefaction technologies have attracted increasing interest during the past few decades.[5-8] Coal to ethylene glycol, as a potentially green and economic coal liquefaction technology, has been attracting extensive attention in both academic and business circles in the past decades. [9-12] Although it is challenging to achieve high industrial production levels, due primarily to achieving good performance of the catalysts, this technology has been scaled-up to industrial levels of production in China and Europe. Until now, China leads the word in this area and successfully built the world's first annual 200 thousand tons coal to ethylene glycol production plant in 2009.[13]
Syngas to ethylene glycol contains several steps and the step of CO oxidative coupling to di-alkyl oxalate is the critical step since di-alkyl oxalate is required for hydrogenation to EG.2CO+2RONO→(COOR)2+2NO  (1)2ROH+2NO+½O2→2RONO+H2O  (2)
Two main chemical reactions are involved in the CO oxidative coupling step, coupling reaction and regeneration reaction, which are shown in Eq. (1) and (2) separately. The reaction in Eq. (1) occurs on supported metal catalysts, where R could be methyl, ethyl or butyl groups. The regeneration reaction shown in Eq. (2) doesn't need any catalyst. Esterification between oxalic acid and alcohol has been employed as a traditional way of synthesizing oxalic ester. However, this method has several problems, such as severe pollution, high energy consumption and high upfront costs. Therefore, oxidative coupling reaction of CO and alkyl nitrite, forming oxalic ester, has been extensively researched in the past decades. [3, 14-20]
Various supported palladium catalysts for gas-phase synthesis of dimethyl oxalate (DMO) or diethyl oxalate (DEO) have been investigated, and the results have demonstrated that higher conversion and selectivity are realized on Pd/α-Al2O3 compared to Pd on active carbon or γ-Al2O3.[21, 22] However, the relatively high Pd loading (around 2 wt %) is always an issue for industrial application of CO oxidative coupling to OMO, which will greatly increase the cost of production. Therefore, the design of low Pd loaded catalysts with high performance is important to industry. A Pd/α-Al2O3 nanocatalyst with ultra-low Pd loading that exhibits high activity and stability for CO oxidative coupling to DMO was developed recently. [23] This catalyst was prepared by a Cu2+ assisted in situ reduction method at room temperature, which significantly increased the dispersion and the specific area of active component Pd, and also decreased the ensemble size of Pd nanoparticles dispersed over the Pd/α-Al2O3. The average size of Pd nanoparticles is 2.7 nm, and the Pd loading could be as low as 0.13 wt %. To further enhance the activity and stability of Pd/α-Al2O3, several metals such as Fe, [24, 25] Ni and Ce were reported as promoters to enhance the dispersion of Pd on the support or decrease the Pd particles size.[24-27] CeO2 was reported as a promoter and in spite of the reaction was evaluated only within 100 min, Pd—CeO2/α-Al2O3 catalyst showed around 20% higher catalytic activity compared to Pd/α-Al2O3 catalyst (without CeO2) for the synthesis of DMO from CO and methyl nitrite. [28]
Although methyl nitrite has been maturely used, especially in China, for the industrial synthesis of DMO, it is controlled in the US due to its highly flammable, highly explosive and toxic properties. Ethyl nitrite, however, is another safe and non-explosive alkyl nitrite that also can be used for CO oxidative coupling reaction.[18, 20, 29-31] Therefore, to find a good catalyst with low Pd loading and high catalytic activity for CO oxidative coupling to DEO is of great significance in the US. Herein, we report a Pd—CeO2/α-Al2O3 nanocatalyst with 0.8% Pd (wt %) loading and 0.2 wt % CeO2 as a catalyst for CO oxidative coupling to DEO. We present the preparation and characterization of two catalysts with and without CeO2 as a promoter. The comparison of catalytic activities between the two catalysts is discussed and the interaction among Pd, ceria and the support leading to the activity differences is also presented.